Rgb monolithic integrated high purity microled display device

ABSTRACT

A colour conversion resonator system, comprising: a first partially reflective region configured to transmit light of a first primary peak wavelength and to reflect light of a second primary peak wavelength; a second partially reflective region configured to at least partially transmit light of the first and second primary peak wavelengths and to reflect light of a third primary peak wavelength; a third partially reflective region configured to at least partially reflect light with the third primary peak wavelength; a first colour conversion resonator cavity arranged to receive input light with the first primary peak wavelength through the first partially reflective region and to convert at least some of the light of the first primary peak wavelength to provide light of the second primary peak wavelength, wherein the first colour conversion resonator cavity is arranged such that the second primary peak wavelength resonates in the first colour conversion resonator cavity and resonant light with the second primary peak wavelength is output through the second partially reflective region; and a second colour conversion resonator cavity arranged to receive input light comprising the second primary peak wavelength through the second partially reflective region and to convert at least some of the second primary peak wavelength to provide light of the third primary peak wavelength, wherein the second colour conversion resonator cavity is arranged such that the third primary peak wavelength resonates in the second colour conversion resonator cavity and resonant light with the third primary peak wavelength is output through the third partially reflective region, wherein the first colour conversion resonator cavity and the second resonator cavity are arranged partially to overlap to provide a non-overlapping portion and an overlapping portion thereby to define a first light emitting surface and a second light emitting surface respectively, wherein the first light emitting surface is arranged to provide resonant light of the second primary peak wavelength and the second light emitting surface is arranged to provide resonant light of the third primary peak wavelength.

FIELD OF THE INVENTION

The invention relates to light emitting diode structures and methods offorming light emitting diode structures. In particular, but notexclusively, the invention relates to a vertically integrated colourconversion resonator system.

BACKGROUND OF THE INVENTION

It is known to generate wavelengths of a desired primary peak wavelengthusing a pump source light emitting diode (LED) to provide input lightand colour conversion materials to convert such input light to light ofa desired wavelength. Such colour conversion materials can be phosphormaterials or quantum dots (QDs), for example. Of particular importanceis to generate light with wavelengths corresponding to red, green andblue light. Such colour light emission has significance in displayapplications.

It is known to provide red, green and blue light from a single wafer ofmonolithically grown light emitting diode devices that produce light ofa particular wavelength (typically blue light), using QD materials todown convert the light. Similarly, red, green and blue light emittingstructures comprising quantum wells (QWs) can be stacked on top of oneanother to produce a stacked device. In such devices, at low currentlevel, the top most QW lights up and by increasing the current level,the middle and bottom QWs are lit up sequentially.

However, QD materials are not ready for microLED display applications assuch materials typically easily degrade over 0.2 W/cm² input power.Further, where QDs are used as the colour conversion material, thethickness of the layers of QD material are typically at least 20 μm inorder to fully absorb input light. Accordingly, the thickness of QDmaterial needed to provide sufficient conversion of light wavelength isgreater than that suitable to provide the pixel size and pitch needed inhigh resolution microLED arrays. Furthermore, typical colour conversionmaterials, such as QDs and phosphor materials, result in a large fullwidth half maximum (FWHM) spectrum and hence a reduced colour gamut.

Accordingly, there is a need for sources of distinct and differentwavelengths of light, such as red, green and blue light, with anincreased colour gamut suitable for microLEDs.

SUMMARY OF THE INVENTION

In order to mitigate for at least some of the above-described problems,there is provided a colour conversion resonator system and a method offorming a colour conversion resonator system in accordance with theappended claims.

There is provided a colour conversion resonator system, comprising: afirst partially reflective region configured to transmit light of afirst primary peak wavelength and to reflect light of a second primarypeak wavelength; a second partially reflective region configured to atleast partially transmit light of the first and second primary peakwavelengths and to reflect light of a third primary peak wavelength; athird partially reflective region configured to at least partiallyreflect light with the third primary peak wavelength; a first colourconversion resonator cavity arranged to receive input light with thefirst primary peak wavelength through the first partially reflectiveregion and to convert at least some of the light of the first primarypeak wavelength to provide light of the second primary peak wavelength,wherein the first colour conversion resonator cavity is arranged suchthat the second primary peak wavelength resonates in the first colourconversion resonator cavity and resonant light with the second primarypeak wavelength is output through the second partially reflectiveregion; and a second colour conversion resonator cavity arranged toreceive input light comprising the second primary peak wavelengththrough the second partially reflective region and to convert at leastsome of the second primary peak wavelength to provide light of the thirdprimary peak wavelength, wherein the second colour conversion resonatorcavity is arranged such that the third primary peak wavelength resonatesin the second colour conversion resonator cavity and resonant light withthe third primary peak wavelength is output through the third partiallyreflective region, wherein the first colour conversion resonator cavityand the second resonator cavity are arranged partially to overlap toprovide a non-overlapping portion and an overlapping portion thereby todefine a first light emitting surface and a second light emittingsurface respectively, wherein the first light emitting surface isarranged to provide resonant light of the second primary peak wavelengthand the second light emitting surface is arranged to provide resonantlight of the third primary peak wavelength.

Preferably, the third partially reflective region is further configuredto reflect light with a fourth primary peak wavelength, the colourconversion resonator system further comprising: a fourth partiallyreflective region configured to at least partially reflect light withthe fourth primary peak wavelength; and a third colour conversionresonator cavity arranged to receive input light comprising the thirdprimary peak wavelength through the third partially reflective regionand to convert at least some of the third primary peak wavelength toprovide light of the fourth primary peak wavelength, wherein the thirdcolour conversion resonator cavity is arranged such that the fourthprimary peak wavelength resonates in the third colour conversionresonator cavity and resonant light with the fourth primary peakwavelength is output through the fourth partially reflective region,wherein the second colour conversion resonator cavity and the thirdcolour conversion resonator cavity are arranged partially to overlap toprovide a non-overlapping portion and an overlapping portion thereby todefine the second light emitting surface and a third light emittingsurface respectively, wherein the second light emitting surface isarranged to provide resonant light of the third primary peak wavelengthand the third light emitting surface is arranged to provide resonantlight of the fourth primary peak wavelength.

Such a configuration forms a monolithic system of epitaxial layers. Incontrast to known monolithic LED devices, the colour conversionresonator system of the present invention is able to provide distinctlight of different wavelengths in a vertically integrated system.Growing such a colour conversion resonator system monolithically removesthe need to use conventional time consuming ‘pick and place’ methodswhereby LEDs are grown individually on a wafer and moved separately ontothe display electronics. Furthermore, the partial overlap createdbetween the first and second colour conversion resonator cavities andthe second and third colour conversion resonator cavities due toselective etching, allows the system to emit light of different colourswith relatively narrow full width half maximum (FWHM) spectra.Furthermore, such a system improves the directionality of light emitted,reducing the need for the integration of collimators or lens, which mayrequire complex processes to implement. Advantageously, improved lightoutput is provided, enabling narrow beam angles and narrow spectra, forexample for use in near eye displays. Beneficially, the colourconversion resonator system enables high colour gamut displays and theformation of high resolution micro LED arrays. Advantageously, theoptical colour conversion resonator system enables wafer levelprocessing and narrow beam angle emission without collimators, andcompressed light emission spectra with reduced efficiency loss.

The colour conversion resonator system can be configured to emit red,green and blue light from the different light emitting surfaces. Such asystem is particularly useful in microLED applications for displayscreens.

Preferably, the first partially reflective region and the secondpartially reflective region are separated by a distance of (N+1)multiplied by λ_(converted)/2n(λ_(converted)), wherein N is an positiveinteger number, λ_(converted) is the second primary peak wavelength andn(λ_(converted)) is the effective refractive index of the materialseparating the first partially reflective region and the secondpartially reflective region, thereby to define the length of the firstcolour conversion resonator cavity and/or wherein the second partiallyreflective region and the third partially reflective region areseparated by a distance of (N+1) multiplied byλ_(converted)/2n(λ_(converted)), wherein N is an positive integernumber, λ_(converted) is the third primary peak wavelength andn(λ_(converted)) is the effective refractive index of the materialseparating the second partially reflective region and the thirdpartially reflective region, thereby to define the length of the secondcolour conversion resonator cavity, and/or wherein the third partiallyreflective region and the fourth partially reflective region areseparated by a distance of (N+1) multiplied byλ_(converted)/2n(λ_(converted)), wherein N is an positive integernumber, λ_(converted) is the fourth primary peak wavelength andn(λ_(converted)) is the effective refractive index of the materialseparating the third partially reflective region and the fourthpartially reflective region, thereby to define the length of the thirdcolour conversion resonator cavity.

Such a configuration enables constructive interference of the secondprimary peak wavelength in the first colour conversion resonator cavity,constructive interference of the third primary peak wavelength in thesecond colour conversion resonator cavity and constructive interferenceof the fourth primary peak wavelength in the third colour conversionresonator cavity. Advantageously, careful tuning of the colourconversion resonator cavity enables enhanced output emission.

Preferably, the colour conversion resonator system further comprises atleast one LED. More preferably, the colour conversion resonator systemcomprises a first LED arranged to control light emission from the firstlight emitting surface and a second LED arranged to control lightemission from the second light emitting surface.

More preferably, the colour conversion resonator system comprises afirst LED arranged to control light emission from the first lightemitting surface, a second LED arranged to control light emission fromthe second light emitting surface and a third LED arranged to controllight emission from the third light emitting surface. Beneficially, sucha system allows each pixel to be controlled individually.

For example, a system with at least three individual LEDs and configuredto emit red, green and blue light can allow only a blue pixel to emitlight, or only a green pixel to emit light or only a red pixel to emitlight. Additionally, a combination of the pixels can emit light suchthat blue and green light is emitted in combination, blue and red lightis emitted in combination, red and green light is emitted in combinationor red, green and blue light is emitted in combination.

Preferably the input light is at least one of ultra violet (UV) lightand blue light, preferably wherein the input light has a wavelength ofbetween 340 nm and 460 nm. Advantageously, high quality, establishedinput LED sources with shorter wavelengths than the wavelength offurther visible light colours required for optical displays are used toprovide an input pump source for the colour conversion in the colourconversion resonator cavity.

Preferably, at least one of the colour conversion resonator cavitiescomprises at least one quantum well layer, preferably wherein the atleast one quantum well layer is placed to coincide with an antinode ofthe colour conversion resonator cavity standing wavelength for convertedlight, thereby enhancing at least one of the intensity, spectral widthand directionality of output light with the resonant convertedwavelength of light.

Alternatively or additionally, there is provided the colour conversionresonator system wherein at least one of the colour conversion resonatorcavities comprises a quantum well layer comprising at least one quantumwell and a further quantum well layer comprising at least one quantumwell, wherein the separation of the quantum well layer and the furtherquantum well layer is N multiplied by λ_(converted)/2n(λ_(converted)),wherein N is an positive integer number, λ_(converted) is the wavelengthof the resonant light in the colour conversion resonator cavity andn(λ_(converted)) is the effective refractive index of the materialbetween the quantum well layer and the further quantum well layer at thewavelength of the resonant light in the colour conversion resonatorcavity.

Beneficially, such a configuration places each quantum well layer at anantinode of the resonant standing wavelength of light in the colourconversion resonator cavities thereby enabling constructive interferenceand enhancement of output light.

Preferably, at least one of the colour conversion resonator cavitiescomprises at least one absorption layer configured to absorb input lightthereby to enable transfer of energy from the input light wavelengthinto the at least one quantum well layer, preferably wherein theabsorption layer comprises a material with a lower energy bandgap thanthe energy of the input light. Advantageously, absorption layers aid theprocess of enabling carriers to recombine in quantum well layers andthus enabling improved resonance of the converted light emitted by thequantum well layers.

Preferably, the colour conversion resonator system further comprises atleast one diffusion barrier arranged to reduce diffusion of carriersfrom at least one of the colour conversion resonator cavities.Advantageously, the use of diffusion barriers reduces diffusion ofcarriers and hence enhances emissive recombination in the colourconversion resonator cavity.

Preferably, the colour conversion resonator system comprises at leastone further partially reflective region corresponding to at least one ofthe first, second or third light emitting surfaces. Advantageously, thepartially reflective regions are tuned in order to optimise whichwavelengths are emitted by light emitting pixels formed by thecombination of colour conversion resonator cavity systems and LEDdevices. Beneficially, light of predefined wavelengths is recycled inthe colour conversion resonator cavities in order to enhance theconversion efficiency of input light with a primary peak wavelength tooutput light with a different primary peak wavelength.

Preferably, at least one of the partially reflective regions and/or thefurther partially reflective regions comprises a distributed Braggreflector (DBR), preferably wherein the DBR is at least one of: a doubleband DBR, a conventional DBR and a vertical stack of two DBRs.

Preferably, at least one of the partially reflective regions comprises ablue wavelength centred low Herpin index distributed Bragg reflector(DBR) or a green wavelength centred low Herpin index DBR or a redwavelength centred low Herpin index DBR.

Preferably, the colour conversion resonator system comprises a bluewavelength centred low Herpin index DBR and a green wavelength centredlow Herpin index DBR and a red wavelength centred low Herpin index DBR.Beneficially, such a configuration creates one pixel optimised for bluewavelength light, one pixel optimised for green wavelength light and onepixel optimised for red wavelength light.

Preferably, at least one of the partially reflective regions and thecolour conversion resonator cavities comprises an epitaxial crystallinelayer, preferably wherein the colour conversion resonator systemcomprises at least one of a dielectric material and a III-Vsemiconductor material. Advantageously, the partially reflective regionis formed using techniques that enable seamless integration of thefunctional layers in the colour conversion resonator cavity.

Preferably, the colour conversion resonator system forms an array ofpixels, wherein the array comprises a first pixel configured to emitlight of a different wavelength to a second pixel and a third pixelconfigured to emit light of a different wavelength to the first pixeland the second pixel. Preferably the first and/or second pixel and/orthird pixel comprises a further partially reflective regioncorresponding to its light emitting surface. Advantageously, lightemitting pixels based on the combination of light emitting devices, suchas LED devices, and colour conversion resonator cavities means that highpurity colour light emitting pixels can be formed on a scale that meansthat they can be implemented in high resolution micro scale arrays.

The colour conversion resonator system is preferably produced by formingat least one of the colour conversion resonator cavities on a substrate,preferably wherein forming at least one of the colour conversionresonator cavities on the substrate comprises epitaxial growth of aplurality of layers. The method further comprising forming at least oneof the partially reflective regions on the substrate, preferably whereinforming at least one of the partially reflective regions on thesubstrate comprises sequentially forming at least one of the colourconversion resonator cavities and partially reflective regions on thesubstrate. The method preferably comprising bonding the colourconversion resonator system to at least one LED and selectively etchingthe colour conversion resonator system, thereby to provide the lightemitting surfaces. Advantageously, forming a colour conversion resonatorcavity on a substrate enables large scale formation of colour conversionresonator cavities for integration with light emitting devices.Beneficially, known growth and processing techniques are applied to formstructures with high quality, low defect density, material that providesfor efficient light input and light conversion for use in light emittingpixels.

Further aspects of the invention will be apparent from the descriptionand the appended claims.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

A detailed description of embodiments of the invention is described, byway of example only, with reference to the figures, in which:

FIG. 1 shows a cross sectional view of a system of three colourconversion resonator cavities;

FIG. 2 shows a cross sectional view of the system of FIG. 1 wherein thecolour conversion resonator cavities are bonded to an LED;

FIG. 3 shows a cross sectional view of the system of three colourconversion resonator cavities of FIG. 2 that has been processed further;

FIG. 4 shows a cross sectional view of the system of three colourconversion resonator cavities of FIG. 3 that has been processed further;

FIG. 5 shows a cross sectional view of the system of three colourconversion resonator cavities of FIG. 4 that has been processed further;

FIG. 6 shows a cross sectional view of a light input and emission from asystem of three colour conversion resonator cavities;

FIG. 7 shows a cross sectional view of a system of three colourconversion resonator cavities with independently addressable lightinput;

FIG. 8 shows a cross sectional view of a system of three colourconversion resonator cavities;

FIG. 9 shows a cross sectional view of the system of three colourconversion resonator cavities of FIG. 8 that has been processed further;

FIG. 10 shows a cross sectional view of the system of three colourconversion resonator cavities of FIG. 9 that has been processed further;and

FIG. 11 shows a cross sectional view of the system of three colourconversion resonator cavities of FIG. 10 that has been processedfurther.

In order to address disadvantages associated with devices in the priorart at least as described above, a structure and method of forming thestructure is described below, with reference to FIGS. 1 to 11 . A colourconversion resonator cavity system is described that provides an elegantway to down convert and reuse input light in an efficient manner, inorder to provide multi-colour wavelength light output systems.Advantageously, such systems provide high purity, narrow FWHM outputlight with narrower beam angles, thereby improving light output controland providing systems with a better colour gamut and controlleddirectionality. Beneficially, the formation and processing ofepitaxially grown crystalline layers can be used to provide highquality, and therefore high efficiency, systems for improved lightoutput. Such epitaxially grown crystalline layers can be used to formthe colour conversion resonator cavity system in a single growth processor groups of one or more epitaxially grown crystalline layers can beoptimised separately and bonded together to form the colour conversionresonator cavity, thereby enabling parallel growth and processing ofseparately optimised layers.

Further, advantageously, the formation and processing of colourconversion resonator cavities formed from epitaxially grown systemsenables the definition of light emitting surfaces associated with theemission of different colours of light, whereby the light emittingsurfaces are associated with pixels that can, advantageously, be formedon a scale suitable for implementation in microLED pixel arrays(including high resolution microLED arrays with light emitting surfacesof pixels less than or equal to 100 μm² and preferably less than orequal to 16 μm², and with pixel pitch less than or equal to 10 μm andpreferably less than or equal to 5 μm).

In FIG. 1 there is shown a cross sectional view of a colour conversionresonator system 100, which is an epitaxial structure that has threecolour conversion resonator cavities. The epitaxial structure is formedand subsequently processed in order to provide a colour conversionresonator system in combination with light input devices, as describedwith reference to FIGS. 2 to 11 .

The colour conversion resonator system 100 is a stack of epitaxialcrystalline compound semiconductor layers. The epitaxial crystallinecompound semiconductor layers are provided by sequential growth of theepitaxial layers on a growth substrate 102. The growth substrate 102,for example a silicon, silicon carbide, sapphire, gallium nitride, orother suitable growth substrate, may be removed after the epitaxialcompound semiconductor crystalline layers have been formed.Beneficially, the growth of such epitaxial compound semiconductorcrystalline layers formed in this manner can be controlled with highprecision to provide high quality material with low defect densities, aswell as controlled thicknesses of layers and efficient emissiverecombination of carriers at controlled wavelengths of light.

The three colour conversion resonator cavities of the epitaxialstructure are each designed to receive input light from one or moreinput light sources and to convert input light with a primary peakwavelength from an input light source to provide output light withdifferent, converted, primary peak wavelengths of light. The epitaxialstructure is designed such that light of the converted primary peakwavelength resonates in its respective colour conversion resonatorcavity of the epitaxial structure and resonant converted light ofmultiple, different, wavelengths is output from the colour conversionresonator system 100 when processed and combined with input lightsources. Appropriate processing of the epitaxial structure enables theprovision of multi-colour emitters, where multiple colour conversionresonator cavities are associated with different light emitting surfacesfor emitting light of different wavelengths, at least as describedherein.

FIG. 1 shows a buffer 104 grown on the growth substrate 102. Thesubstrate 102 is a silicon substrate and the buffer 104 an aluminiumgallium nitride (AlGaN) epitaxial layer. In further examples,alternatively or additionally, the buffer 104 is formed of at least oneof aluminium gallium nitride (AlGaN), aluminium nitride (AlN), andgallium nitride (GaN).

Upon the buffer 104 there is grown an etch stop 106. The etch stop 106is an AlGaN layer with relatively high aluminium content. The etch stop106 facilitates accurate control of processing steps that are used toremove material from the epitaxial structure 100 in order to provide aprocessed system.

Atop the etch stop 106 there is grown a partially reflective region 108.Upon the partially reflective region 108 there is grown a colourconversion resonator cavity 110 and a further partially reflectiveregion 112. The colour conversion resonator cavity 110 is configured toreceive input light of a primary peak wavelength and convert this inputlight to converted light of a different primary peak wavelength.

Atop the partially reflective region 112 there is grown a further colourconversion resonator cavity 114 and a further partially reflectiveregion 116. The colour conversion resonator cavity 114 is configured toreceive input light of another primary peak wavelength and convert thisinput light to converted light of a different primary peak wavelength.In a further example, an etch stop layer is formed between the partiallyreflective region 112 and the further colour conversion resonator cavity114. The etch stop layer (not shown) facilitates close control insubsequent steps to remove material from the structure. In furtherexamples, alternative or additional etch stop layers are formed withinthe structure in order to facilitate control of removal of layers byetching processes.

Atop the partially reflective region 116 there is grown a further colourconversion resonator cavity 118 and a further partially reflectiveregion 120. The colour conversion resonator cavity 118 is configured toreceive input light of a further primary peak wavelength and convertthis input light to converted light of a different primary peakwavelength. In a further example, an etch stop layer is formed betweenthe partially reflective region 116 and the further colour conversionresonator cavity 118. The etch stop layer (not shown) facilitates closecontrol in subsequent steps to remove material from the structure. Infurther examples, alternative or additional etch stop layers are formedwithin the structure in order to facilitate control of removal of layersby etching processes.

Colour conversion resonator system 100 forms a monolithic system ofepitaxial layers. Such epitaxial layers are planar layers. The colourconversion resonator system 100 of FIG. 1 is formed using epitaxialcompound semiconductor growth techniques such as metalorganic chemicalvapour deposition (MOCVD) and molecular beam epitaxy (MBE).Additionally, or alternatively, the colour conversion resonator system100 is formed using any appropriate technique.

The order of the epitaxial layers is sequentially grown such that whenthe system is flipped and bonded to an LED, the order of proximity ofthe colour conversion resonator cavities with respect to the LED is suchthat shorter wavelength light, such as UV light, from the LED isabsorbed in the colour conversion resonator cavity 118 and then longerwavelength light, such as blue light, is output from the colourconversion resonator cavity 118. The light output from the colourconversion resonator cavity 118 and the LED is absorbed in the colourconversion resonator cavity 114. The colour conversion resonator cavity114 then outputs light with a longer wavelength than the LED and thecolour conversion cavity 118, such as green light. Light output from thecolour conversion resonator cavity 118, the colour conversion resonatorcavity 114 and the LED is absorbed in the colour conversion resonatorcavity 110, which then outputs yet longer wavelength light, such as redlight. This means that input light can be absorbed and emitted by thecolour conversion resonator cavities in such a way that the light thatis emitted by the successive colour conversion resonator cavities isreused before exiting the eventual structure.

Advantageously, growing the epitaxial structure of the colour conversionresonator system 100 in this order means that the colour conversionresonator cavities 118, 114, 110 can be handled using the growthsubstrate 102 upon which the layers of the colour conversion resonatorsystem 100 are formed, in order to facilitate bonding with LEDstructures without requiring further processing steps to enablealignment and bonding of the colour conversion resonator system 100 withone or more light emitting devices formed on a different underlyingsubstrate.

The colour conversion resonator system 100 described with respect toFIG. 1 is formed from nitride-based materials. In particular, theepitaxial crystalline compound semiconductor layers are Gallium Nitride(GaN) based materials. Whilst the structures described in relation toFIG. 1 relate to nitride-based semiconductor compound materials, theskilled person understands that the concepts described herein areapplicable to other materials, in particular to other semiconductormaterials, for example other III-V compound semiconductor materials, orII-VI compound semiconductor materials.

The provision of three colour conversion resonator cavities enablesstructures that emit multiple, different, primary peak wavelengths oflight to be formed. The skilled person understands that alternative oradditional structures are used in further examples in order to providedifferent structures that emit different primary peak wavelengths oflight.

The partially reflective region 108 and the further partially reflectiveregion 112 are separated by a distance of (N+1) multiplied byλ_(converted)/2n(λ_(converted)) wherein N is an positive integer number,λ_(converted) is the converted primary peak wavelength emitted from thecolour conversion resonator cavity 110 and n(λ_(converted)) is theeffective refractive index of the material separating the partiallyreflective region 108 and the further partially reflective region 112.Such a configuration allows light of the converted primary peakwavelength to resonate in the colour conversion resonator cavity 110. Infurther examples, the partially reflective region 108 and the furtherpartially reflective region 112 are separated by a different distance.

Similarly, the partially reflective region 112 and the further partiallyreflective region 116 are separated by a distance of (N+1) multiplied byλ_(converted)/2n(λ_(converted)) wherein N is an positive integer number,λ_(converted) is the converted primary peak wavelength emitted from thecolour conversion resonator cavity 114 and n(λ_(converted)) is theeffective refractive index of the material separating the partiallyreflective region 112 and the further partially reflective region 116.Such a configuration allows light of the converted primary peakwavelength to resonate in the colour conversion resonator cavity 114. Infurther examples, the partially reflective region 112 and the furtherpartially reflective region 116 are separated by a different distance.

Additionally, the partially reflective region 116 and the furtherpartially reflective region 120 are separated by a distance of (N+1)multiplied by λ_(converted)/2n(λ_(converted)), wherein N is an positiveinteger number, λ_(converted) is the converted primary peak wavelengthemitted from the colour conversion resonator cavity 118 andn(λ_(converted)) is the effective refractive index of the materialseparating the partially reflective region 116 and the further partiallyreflective region 120. Such a configuration allows light of theconverted primary peak wavelength to resonate in the colour conversionresonator cavity 118. In further examples, the partially reflectiveregion 116 and the further partially reflective region 120 are separatedby a different distance.

The colour conversion resonator cavity 110 comprises at least onequantum well layer. The quantum well layer comprises multiple quantumwells. In further examples, the quantum well layer comprises a singlequantum well. The quantum well layer is placed at an antinode of thecolour conversion resonator cavity standing wavelength for the convertedprimary peak wavelength emitted from the colour conversion resonatorcavity 110. Similarly, the colour conversion resonator cavity 114comprises at least one quantum well layer, placed at an antinode of thecolour conversion resonator cavity standing wavelength for the convertedprimary peak wavelength emitted from the colour conversion resonatorcavity 114. The quantum well layer comprises multiple quantum wells. Infurther examples, the quantum well layer comprises a single quantumwell. Additionally, the colour conversion resonator cavity 118 comprisesat least one quantum well layer, placed at an antinode of the colourconversion resonator cavity standing wavelength for the convertedprimary peak wavelength emitted from the colour conversion resonatorcavity 118. The quantum well layer comprises multiple quantum wells. Infurther examples, the quantum well layer comprises a single quantumwell. Such a configuration enhances at least one of the intensity,spectral width and output light with the resonant converted primary peakwavelengths. In further examples, the colour conversion resonatorcavities 110, 114, 118 each have alternative or additional layers, forexample single or multiple quantum wells in the quantum well layers arepositioned to coincide with different antinodes of the convertedwavelength of light in the respective colour conversion resonatorcavities 110, 114, 118.

The colour conversion resonator cavities 110, 114 and 118 each comprisemultiple quantum wells (MQWs). In further examples, the colourconversion resonator cavities 110, 114, 118 each comprise a singlequantum well (SQW). In further examples, the colour conversion resonatorcavities 110, 114, 118 comprise different layers from one another. Thequantum well layers are designed to enable carriers to recombine suchthat emissive recombination results in an output of light with a primarypeak wavelength that is different to the wavelength of input light thatresults in the emission of the output light.

In order to enable emission, input light is absorbed by absorptionlayers associated with respective quantum well layers in each of thecolour conversion resonator cavities 110, 114, 118. The input lightabsorbed at the absorption layers has a primary peak wavelength. In anexample, the input light is blue light with a wavelength ofapproximately 450 nm. The wavelength of light output by the quantum welllayers is longer than the wavelength input. The output wavelength oflight is the converted wavelength of light. Whilst the input light isblue light, in further examples, additional or alternative wavelengthsof input light are used. More preferably, each absorption layercomprises material with a lower energy bandgap than the energy of theinput primary peak wavelength.

The epitaxial structure of the colour conversion resonator system 100,once formed, is designed to be inverted and bonded to light emittingdevices and the substrate 102, buffer 104 and etch stop 106 removed.Accordingly, the order of the sequence of layers of input light andsubsequent, converted, output light in the epitaxial structure of thecolour conversion resonator system 100 is considered before growth andthe formation of the partially reflective regions is described in moredetail with respect to FIGS. 2 and 3 , below. The partially reflectiveregions 108, 112, 116 and 120 are Distributed Bragg Reflectors (DBRs).In further examples, the partially reflective regions 108, 112, 116 and120 comprise alternative or additional structures.

Once the system 100 has been formed, it is inverted and bonded to an LEDstructure. This is shown at FIG. 2 . In FIG. 2 there is shown a colourconversion resonator system 200 that comprises a light emitting diode(LED) 224 and substrate device 202. The substrate device is a temporarysubstrate used to facilitate the processing of the colour conversionresonator system 200. Alternatively, in an example, the substrate device202 is a complementary metal oxide semiconductor (CMOS) backplane thatis combined with light emitting devices, such as light emitting diodedevices, in order to provide and control input light in the eventualstructure. The colour conversion resonator system 100 described withrespect to FIG. 1 is combined with the substrate device 202 and the LED224 by inverting the colour conversion resonator structure 100 describedwith reference to FIG. 1 and bonding the uppermost partially reflectiveregion 120 epitaxial layer to the LED 224 using a bonding layer 222. Thesubstrate 102, buffer, 104 and etch stop 106 are then removed, leavingthe structure shown at FIG. 2 .

Advantageously, the order of proximity of the colour conversionresonator cavities with respect to the LED 224 is such that shorterwavelength light, such as blue light, from the LED 224 is absorbed inthe first colour conversion resonator cavity, then light output from thefirst colour conversion resonator cavity and the LED 224 is absorbed inthe second colour conversion resonator cavity, and light output from thefirst colour conversion resonator cavity, the second colour conversionresonator cavity and the LED 224 is absorbed in the third colourconversion resonator cavity. This means that input light can be absorbedand emitted by the colour conversion resonator cavities in such a waythat the light that is emitted by the colour conversion resonatorcavities is reused before exiting the eventual structure.

Accordingly the partially reflective region 120 is shown directly atopthe bonding layer 222 on the LED 224. On top of the partially reflectiveregion 120 is the colour conversion resonator cavity 118 followed by thefurther partially reflective region 116. On top of the partiallyreflective region 116 is the further colour conversion resonator cavity114 followed by the further partially reflective region 112. On top ofthe partially reflective region 112 is the further colour conversionresonator cavity 110 followed by the further partially reflective region108.

Whilst the colour conversion resonator system 100 of FIG. 1 is processedin order to bond to the LED 224, as shown in FIG. 2 , in furtherexamples, the epitaxial layers of the colour conversion resonator system200 are grown directly on the LED 224. Beneficially, such direct growthof layers atop the LED 224 prevent later bonding steps in themanufacture of such devices.

Whilst the epitaxial layers of the colour conversion resonator system100 described with reference to FIG. 1 are shown to be grown in aparticular order in order to enable bonding of the structure to a lightemitting diode structure, in further examples, the order of growth isreversed in order to preserve the efficient absorption and emission oflight from shorter to longer wavelengths from the colour conversionresonator cavity nearest to the input LED light source to the colourconversion resonator cavity furthest from the input LED.

In FIG. 2 there is shown a colour conversion resonator system 200. Asdescribed above, the colour conversion resonator system 200 is formed byinverting the colour conversion resonator system 100 and bonding thecolour conversion resonator system 100 to the LED 224 via the bondinglayer 222 such that the partially reflective region 120 is bondeddirectly atop the LED 224 and the bonding layer 222, and such that thepartially reflective region 108 is at the top of the colour conversionresonator system 200 and subsequently removing the substrate 102 and thebuffer 104 from the colour conversion resonator system 200. The colourconversion resonator system 100 may be inverted by handling thesubstrate 102 and the buffer 104 of the colour conversion resonatorsystem 100 before removing said layers.

The LED 224 is bonded to the partially reflective region 120 usingdielectric bonding. The surface of the LED 224 that is to be bonded tothe partially reflective region 120 is terminated with a high densityoxide film in order to facilitate such bonding. The surface of thepartially reflective region 120 that is to be bonded to the input LED224 is also terminated with a high density oxide film in order tofacilitate wafer level oxide bonding. Accordingly, the primary lightemitting surface of the LED 224 is placed in close proximity or contactwith the partially reflective region 120 such that light that is outputfrom the LED 224 acts as input light for the colour conversion resonatorsystem 200.

In further examples, the LED 224 is bonded to the partially reflectiveregion 120 using polymer bonding, such as polyimide bonding. In furtherexamples, additional or alternative bonding mechanisms are used in orderto attach the LED 224 to the partially reflective region 120.Advantageously, the LED 224 is bonded to the partially reflective region120 to form a single device with minimal interface loss of lightemission from the LED 224 at the interface with the colour conversionresonator system 200.

The colour conversion resonator system 200 is configured to receiveinput light of a first primary peak wavelength and convert this inputlight to light of a second primary peak wavelength. The colourconversion resonator system 200 further converts light of the secondprimary peak wavelength (and light of the first primary peak wavelength)to light of a third primary peak wavelength. Light of the third primarypeak wavelength (and the first and second primary peak wavelengths) isthen converted to light of a fourth primary peak wavelength.

Such a set up allows the colour conversion resonator cavity 118 toreceive input light of the first primary peak wavelength from the LED224 before the further colour conversion resonator cavities 114 and 110.This is efficient when the colour conversion resonator cavity 118 isconfigured for resonant light of the second primary peak wavelengthwherein this wavelength is less than the third primary peak wavelengthand the fourth primary peak wavelength. The third primary peakwavelength is greater than the second primary peak wavelength and lessthan the fourth primary peak wavelength.

For example, the colour conversion resonator cavity 118 can be optimisedfor a wavelength of light corresponding to blue light (e.g.,approximately 450 nm whereby the input light has a shorter wavelength,e.g., UV light at approximately 380 nm), the further colour conversionresonator cavity 114 can be optimised for a wavelength of lightcorresponding to green light (e.g., approximately 530 nm, whereby theinput light has a shorter wavelength, e.g., blue light and UV light),and the further colour conversion resonator cavity 110 can be optimisedfor a wavelength of light corresponding to red light (e.g.,approximately 630 nm, whereby the input light has a shorter wavelength,e.g., green light, blue light and UV light).

In order to enable resonance of wavelengths of light, the partiallyreflective regions 108, 112, 116, 120 are configured to improve thepassage of light through the colour conversion resonator system 200 fromthe light input LED 224 to a light emitting surface.

The partially reflective region 120 has a relatively high reflectivityfor the wavelength of converted light generated in the colour conversionresonator cavity 110 and a relatively high transmissivity for thewavelength of the input light. In an example, the partially reflectiveregion 120 has a relatively low reflectivity, e.g., less than 20% of theprimary peak wavelength of the input light from the LED 224 bonded tothe partially reflective region 120 and a relatively high reflectivity,e.g., more than 80%, of converted light generated by absorption of theinput light and re-emission in the colour conversion resonator cavity118. In further examples, different reflectivity values are used for thepartially reflective region 120. In an example, the partially reflectiveregion 120 has a reflectivity of input light of less than 10% and areflectivity of converted light of greater than 90%. In a furtherexample, the partially reflective region 120 has a reflectivity of inputlight of less than 5% and a reflectivity of converted light of greaterthan 95%. Similarly, the partially reflective region 116 has arelatively high reflectivity for the wavelength of converted lightgenerated in the colour conversion resonator cavity 114 and a relativelyhigh transmissivity for the wavelength of the input light.

In an example, the partially reflective region 116 has a relatively lowreflectivity, e.g., less than 20% of the primary peak wavelength of theinput light and a relatively high reflectivity, e.g., more than 80%, ofconverted light generated by absorption of the input light andre-emission in the colour conversion resonator cavity 114. In furtherexamples, different reflectivity values are used for the partiallyreflective region 116. In an example, the partially reflective region116 has a reflectivity of input light of less than 10% and areflectivity of converted light of greater than 90%. In a furtherexample, the partially reflective region 116 has a reflectivity of inputlight of less than 5% and a reflectivity of converted light of greaterthan 95%. Additionally, the partially reflective region 112 has arelatively high reflectivity for the wavelength of converted lightgenerated in the colour conversion resonator cavity 110 and a relativelyhigh transmissivity for the wavelength of the input light.

In an example, the partially reflective region 112 has a relatively lowreflectivity, e.g., less than 20% of the primary peak wavelength of theinput light and a relatively high reflectivity, e.g., more than 80%, ofconverted light generated by absorption of the input light andre-emission in the colour conversion resonator cavity 110. In furtherexamples, different reflectivity values are used for the partiallyreflective region 112. In an example, the partially reflective region112 has a reflectivity of input light of less than 10% and areflectivity of converted light of greater than 90%. In a furtherexample, the partially reflective region 112 has a reflectivity of inputlight of less than 5% and a reflectivity of converted light of greaterthan 95%.

In an example, the partially reflective region 108 has a relatively lowreflectivity, e.g., less than 20% of the primary peak wavelength of theinput light and a relatively high reflectivity, e.g., more than 80%, ofconverted light generated by absorption of the input light andre-emission in the colour conversion resonator cavity 110. In furtherexamples, different reflectivity values are used for the partiallyreflective region 108. In an example, the partially reflective region108 has a reflectivity of input light of less than 10% and areflectivity of converted light of greater than 90%. In a furtherexample, the partially reflective region 108 has a reflectivity of inputlight of less than 5% and a reflectivity of converted light of greaterthan 95%.

The partially reflective regions 108, 112, 116 and 120 are formed fromalternating epitaxial crystalline layers of different refractiveindices. The refractive indices of the layers, and the thicknesses ofthe layers, are selected in order to provide a reflectivity response asa function of the wavelength of light incident at the partiallyreflective regions 108, 112, 116 and 120. Growth of a DBR in this mannerenables seamless formation of a high crystalline quality colourconversion resonator system 100.

Whilst the partially reflective regions 108, 112, 116 and 120 are DBRs,in further examples, alternative or additional regions are used. In afurther example, the partially reflective region 108 comprises a DBR ora vertical stack of two different DBRs or a double band DBR. In afurther example, the partially reflective region 108 is omitted. In afurther example, the partially reflective region 112 and/or thepartially reflective region 116 and/or the partially reflective region120 comprise a DBR with relatively high reflectivity of convertedwavelengths of light and low reflectivity of input wavelengths. orexample, as a filter for high reflectivity of blue light and lowreflectivity of green and red light, or as a filter for low reflectivityof blue light and high reflectivity of green and red light. Reflectivityof light either side of a range of wavelengths may also be implemented.Where defines a quarter wavelength thick, high refractive indexmaterial, defines a quarter wavelength thick, low refractive indexmaterial, for N layers, a

$\left( {\frac{L}{2}H\frac{L}{2}} \right)^{N}{and}\left( {\frac{H}{2}L\frac{H}{2}} \right)^{N}$

stack can be used to suppress the reflectivity of the short and longwavelength side, respectively, a

$\left( {\frac{H}{2}L\frac{H}{2}} \right)^{N}$

stack can be used as a filter for high reflectivity of blue light andlow reflectivity of green and red light and a

$\left( {\frac{L}{2}H\frac{L}{2}} \right)^{N}$

stack can be used as a filter for low reflectivity of blue light andhigh reflectivity of green and red light. In other examples, otherarrangements are used selectively to filter light.

Whilst the partially reflective regions 108, 112, 116 and 120 are DBRsformed of nitride-based epitaxial layers, in further examples thepartially reflective regions 108, 112, 116 and 120 are additionally, oralternatively formed using different methods whilst maintaining thefunctionality of enabling reflection of some wavelengths of light andtransmission of different wavelengths of light. For example, thepartially reflective region 108 and/or the partially reflective region112 and/or the partially reflective region 116 and/or the partiallyreflective region 120 are/is formed from dielectric stacks. In a furtherexample, the partially reflective region 108 and/or the partiallyreflective region 112 and/or the partially reflective region 116 and/orthe partially reflective region 120 are/is formed from alternatinglayers of GaN and porous GaN. The porosity of the epitaxial crystallineGaN layers forming the partially reflective regions 108, 112, 116 and120 is controlled in order to provide the desired reflectivity responseas a function of wavelength, since the porosity of the epitaxialcrystalline layers is linked to their refractive index. Advantageously,DBRs formed in this manner can be provided using GaN alone.

Preferably, there is provided at least one diffusion barrier arranged toreduce diffusion of carriers from the colour conversion resonatorcavities 110, 114 and 118. Diffusion barriers are incorporated in thestructure in order to enhance resonant light emission of converted lightin the colour conversion resonator cavities.

In FIG. 3 there is shown a colour conversion resonator system 300comprising the LED 224, the bonding layer 222, the partially reflectiveregion 120, the colour conversion resonator cavity 118, the furtherpartially reflective region 116 and the further colour conversionresonator cavity 114. Each of these layers is grown sequentially asdescribed above (e.g., either in the order shown in FIG. 1 and flipped,or in the order shown in FIG. 2 , without being flipped). Atop thisseries of layers partially etched layers from the structure describedwith respect to FIGS. 1 and 2 is shown. The etched layers are thepartially reflective region 112, the colour conversion resonator cavity110 and the partially reflective region 108. These layers have beenetched such that said layers 108, 110 and 112 form a partiallyoverlapping region with remaining layers 224, 222, 120, 118, 116 and114.

The colour conversion resonator system 300 is formed by selectivelyetching the colour conversion resonator system 200 in a first region.The surface of the partially reflective region 108 is selectivelypatterned according to known techniques. Such selective patterningallows for selective etching of regions of the colour conversionresonator system (e.g., using known wet or dry etching techniques). Asshown at FIG. 3 , a first etch has removed the partially reflectiveregions 108, 112 and the colour conversion resonator cavity 110 from thecolour conversion resonator system 200 in the first region. The use ofan etch stop (not shown) between the colour conversion resonator cavity114 and the partially reflective region 112 facilitates control of theremoval of material by etching. In further examples, additionally oralternatively, the partially reflective region 112 is not removed duringthe first etch process. The first etch process forms a light emittingsurface region associated with the partially reflective region 108.Whilst one region is shown as the partially reflective region 108, infurther examples, multiple regions are etched to provide light emittingsurfaces associated with the partially reflective region 108. Suchmultiple regions are used to form arrays.

Once the first etch process has been performed in order selectively toremove material associated with the colour conversion resonator cavity110, a second etch process is performed. This is shown at FIG. 4 .

In FIG. 4 there is shown a colour conversion resonator system 400comprising the LED 224, the bonding layer 222, the partially reflectiveregion 120 and the colour conversion resonator cavity 118. These layers,grown sequentially, as described above, remain unetched. Atop saidlayers 224, 222, 120 and 118, is the partially reflective region 116 andthe further colour conversion resonator cavity 114. The partiallyreflective region 116 and the colour conversion resonator cavity 114have selectively been etched such that layers 114 and 116 form apartially overlapping region with remaining layers 224, 222, 120 and118. As described above, atop layers 114 and 116 is the furtherpartially reflective region 112, the further colour conversion resonatorcavity 110 and the further partially reflective region 108 such thatsaid layers 112, 110 and 108 form a partially overlapping region withlayers 114 and 116. The use of an etch stop (not shown) between thecolour conversion resonator cavity 118 and the partially reflectiveregion 116 facilitates control of the removal of material by etching. Infurther examples, additionally or alternatively, the partiallyreflective region 116 is not removed during the second etch process. Thesecond etch process forms an exposed light emitting surface regionassociated with the colour conversion resonator cavity 114 and anexposed light emitting surface region associated with the colourconversion resonator cavity 118. Whilst the etch process is describedwith respect to a cross-sectional view of three exposed regionsassociated with different layers of colour conversion resonatorcavities, in further examples, multiple regions are etched to providelight emitting surfaces associated with different layers of colourconversion resonator cavities in order to form arrays, with twodimensional arrays of light emitting pixels, each light emitting pixelhaving an associated light emitting surface.

The colour conversion resonator system 400 is formed by etching thecolour conversion resonator system 300 in a second region. A second etchhas removed the partially reflective region 116 and the colourconversion resonator cavity 114 from the colour conversion resonatorsystem 300 in the second region.

Beneficially, such a system creates a colour conversion resonator system400 with light emitting surfaces associated with different regions,where the light emitting surfaces are provided by the exposed regionsand enable light of three different primary peak wavelengths to beemitted from the colour conversion resonator system 400.

In FIG. 5 there is shown a colour conversion resonator system 500 wherethe colour conversion resonator system 400 described with reference toFIG. 4 has been further processed to provide a first further partiallyreflective region 526, a second further partially reflective region 528,and a third further partially reflective region 530. The third furtherpartially reflective region 530 associated with the colour conversionresonator cavity 110 is provided instead of the partially reflectiveregion 108 formed in the initial epitaxial structure. Alternatively, thepartially reflective region 108 remains in place and the third furtherpartially reflective region 530 is not formed in the structure shown atFIG. 5 . The first further partially reflective region 526 is formedatop the exposed surface of the colour conversion resonator cavity 118.The second further partially reflective region 528 is formed atop theexposed surface of the colour conversion resonator cavity 114.

The partially reflective regions 108, 112, 116 and 120 and/or thefurther partially reflective regions 526, 528, 530 comprise aDistributed Bragg Reflector (DBR). Such a DBR is preferably one of adouble band DBR, a conventional DBR and a vertical stack of two DBRs.More preferably, the partially reflective regions 112, 116 and 120comprise a low Herpin index DBR whilst the partially reflective region108 and the further partially reflective regions 526, 528 and 530comprise a double band DBR, a conventional DBR and a vertical stack oftwo DBRs.

In an example, the partially reflective regions 108, 112, 116 and 120comprises a blue wavelength centred low Herpin index DBR, or a greenwavelength centred low Herpin index DBR, or a red wavelength centred lowHerpin index DBR. For example, the partially reflective region 120 mayhave a blue wavelength centred low Herpin index DBR such that the firstpixel is optimised for blue wavelength light. The partially reflectiveregion 116 may have a green wavelength centred low Herpin index DBR suchthat the second pixel is optimised for green wavelength light. Thepartially reflective region 112 may have a red wavelength centred lowHerpin index DBR such that the third pixel is optimised for redwavelength light.

Such a configuration provided by the colour conversion resonator system500 enables light emitting surfaces to be provided in order to formarrays of pixels. The etching and deposition of partially reflectiveregions described above results in the creation of a first pixel withthe first further partially reflective region 526 as a top layer, asecond pixel with the second further partially reflective region 528 asa top layer and a third pixel with the third further partiallyreflective region 530 as a top layer. The first pixel has a pixeldimension 532. The second pixel has a pixel dimension 534. The thirdpixel has a pixel dimension 536. Whilst the pixel dimensions 532, 534,534 are shown in cross section, the skilled person understands that thein plan view the pixels have exposed light emitting surfaces associatedwith the dimensions 532, 534, 536 (for example, pixels with square lightemitting surface areas—in further examples, pixels of different forms ofarrays and shaped light emitting surfaces are formed). Further, whilstthe first and second partially reflective regions 526, 528 are shown toabut partially reflectively regions 116 and 112 respectively, in furtherexamples, the first and second partially reflective regions 526 528 havedifferent surface coverage. Further, whilst the relative thicknesses ofthe cross sectional image are shown in the Figures, the skilled personunderstands that in further examples the layers have different relativedimensions.

In FIG. 6 there is shown a colour conversion resonator system 600showing the colour conversion resonator system 500 bonded to a CMOSbackplane 602 further showing input light of the first primary peakwavelength 742, converted light of the second primary peak wavelength744, converted light of the third primary peak wavelength 746 andconverted light of the fourth primary peak wavelength 748.

The partially reflective region 120 is designed such that light of thefirst primary peak wavelength 742 is transmitted and light of the secondprimary peak wavelength 744 is reflected. The partially reflectiveregion 116 is configured such that light of the first primary peakwavelength 742 and light of the second primary peak wavelength 744 ispartially transmitted and light of the third primary peak wavelength 746is reflected. The partially reflective region 112 is configured suchthat light of the first primary peak wavelength 742, light of the secondprimary peak wavelength 744 and light of the third primary peakwavelength 746 is partially transmitted and light of the fourth primarypeak wavelength 748 is reflected.

Input light of the first primary peak wavelength 742 is emitted from theLED 224 through the colour conversion resonator system 700. In theexample of FIG. 6 , the first primary peak wavelength 742 corresponds toUV light. Light of the first primary peak wavelength 742 is transmittedthrough the partially reflective region 120 into the colour conversionresonator cavity 118, where the light is absorbed and down converted byemissive recombination. Light of the first primary peak wavelength 742is converted in the colour conversion resonator cavity 118 to light ofthe second primary peak wavelength 744. In the example of FIG. 6 , lightof the second primary peak wavelength corresponds to blue light.

When an LED, such as the light emitting device 224, is coupled with thecolour conversion resonator system 600, the angular distribution oflight emission of the input LED 224 is altered. Once the input lightfrom an LED 224 with such a Lambertian distribution of light emissionhas been absorbed in the MQWs and pump absorbing layers of the colourconversion resonator cavity 118, electron hole pairs are generated inthe MQWs and pump absorbing layers. The electrons and holes generated inthe pump absorbing layers move to the MQWs. Therefore, the emitted lightwavelength is determined by MQW transitions wavelength. This transitionwavelength has a spectral range (FWHM: full width half maximum) of ˜30nm for green and ˜50 nm for Red when QW materials are AlxInyGa1−x−yN. Ingeneral, AlxInyGa1−x−yN or AlxInyGa1−x−yP MQWs emit the light alldirections but the colour conversion cavity resonator enhances theemission meeting the cavity condition. The results are narrow beam angleand concentrated emission spectrum of the light of the second primarypeak wavelength 744 that is emitted from the colour conversion resonatorsystem 600 of FIG. 6 . Similar absorption and transmission occurs in theother colour conversion resonator cavities 114, 110 in accordance withtheir respective absorption and emission properties.

Light of the second primary peak wavelength 744 resonates in the colourconversion resonator cavity 118 and is transmitted at least in partthrough the partially reflective region 116. Light of the second primarypeak wavelength 744 is also transmitted through the first furtherpartially reflective region 526 and emitted from the associated lightemitting surface.

The relative properties of the partially reflective regions 116, 526 onthe colour conversion resonator cavity 118 are such that resonantconverted light 744 is emitted from a first pixel associated (e.g.,pixel with dimension 532 of FIG. 5 ) with the partially reflectiveregion 526 and such that any converted light with the second primarypeak wavelength 744 and light with the first primary peak wavelengthpass through the partially reflective region 116 such that the light inparts of the colour conversion resonator system 600 are efficientlyreused.

Accordingly, at regions associated with a second and a third pixel(e.g., the regions associated with pixel dimensions 534 and 536 of FIG.5 ), light of the second primary peak wavelength 744 is received in thecolour conversion resonator cavity 114 through the partially reflectiveregion 116. Light of the first primary peak wavelength 742 that has notbeen converted is also received in the colour conversion resonatorcavity 114.

Light of the first primary peak wavelength 742 and light of the secondprimary peak wavelength 744 is at least partially converted in thecolour conversion resonator cavity 114 to light of the third primarypeak wavelength 746. In the example of FIG. 6 , the third primary peakwavelength corresponds to green light.

Light of the third primary peak wavelength 746 resonates in the colourconversion resonator cavity 114 and is transmitted through the partiallyreflective region 112. Light of the third primary peak wavelength 746 isalso transmitted through the second further partially reflective region528 and emitted.

At a second pixel (e.g., the pixel associated with pixel dimension 534of FIG. 5 ), light of the third primary peak wavelength 746 istransmitted through the second further partially reflective region 528and emitted. At a third pixel (e.g., the pixel associated with pixeldimension 536 of FIG. 5 ), light of the first primary peak wavelength742, second primary peak wavelength 744 and third primary peakwavelength 746 is received in the colour conversion resonator cavity 110through the partially reflective region 112. Light of the third primarypeak wavelength 746 is converted in the colour conversion resonatorcavity 110 to light of the fourth primary peak wavelength 748. Light ofthe fourth primary peak wavelength 748 corresponds to red light.

Light of the fourth primary peak wavelength 748 resonates in the colourconversion resonator cavity 110 and is transmitted through the partiallyreflective region 108 and/or the third further partially reflectiveregion 530 to be emitted.

Preferably, input light of the first primary peak wavelength 742 has awavelength corresponding to ultraviolet (UV) wavelength light.Alternatively, or additionally, input light of the first primary peakwavelength 742 has a wavelength corresponding to blue light. In furtherexamples, different wavelengths of light are used.

Whilst a system showing colour conversion to provide blue, green and redconverted light output is demonstrated, in further examples, blue lightis used as the first primary peak wavelength. Advantageously, one of thecolour conversion resonator cavities and the associated partiallyreflective layers need not be used where red, green and blue lightoutputs are desired.

Input light of the first primary peak wavelength 742 has a wavelengthcorresponding to UV wavelength light. The converted light of the secondprimary peak wavelength 744 corresponds to blue wavelength light suchthat the first pixel emits light of the colour blue.

The converted light of the third primary peak wavelength 746 correspondsto green wavelength light such that the second pixel emits light of thecolour green. The converted light of the fourth primary peak wavelength748 corresponds to red wavelength light such that the third pixel emitslight of the colour red. Such an embodiment allows for a monolithicintegration of red, green and blue pixels to provide a monolithic colourconversion system.

In examples, the first pixel, the second pixel and the third pixel areisolated and individually addressable by the CMOS backplane 602, therebyenabling the formation of a multicolour light emitting display.

Whilst FIG. 6 illustrates the LED 224, in further examples, individuallight emitting diodes are used selectively to provide light to lightemitting surfaces associated with particular colour conversion resonatorcavities and associated output light. In FIG. 7 there is shown analternative embodiment of a colour conversion resonator system 700. Thecolour conversion resonator system 700 comprises the first LED 224, asecond LED 638 and a third LED 640. The LEDs 224, 638 and 640 are placedadjacent to one another. Atop the LEDs 224, 638 and 640 are layers 222,120, 118, 116, 114, 112, 110, 108, 526, 528 and 530 sequentially grownand in the selectively etched configuration of colour conversionresonator system 500.

The first LED 224 is bonded such that input light from the first LED 224is provided to the first pixel with the pixel dimension 532, the secondLED 638 is bonded such that input light from the second LED 638 isprovided to the second pixel with the pixel dimension 534, the third LED640 is bonded such that input light from the third LED 640 is providedto the third pixel with the pixel dimension 536. The LEDs 224, 638, 640are bonded to the colour conversion resonator cavity system inaccordance with the techniques described herein with reference to FIGS.1 to 6 . The LEDs 224, 638, 640 are individually addressable LED devicesthat can be addressed using a suitable backplane, such as a Si basedCMOS backplane.

Beneficially the colour conversion resonator system 700 allowscontrolled light emission from each of the three pixels individually.The improved angular distribution, intensity and colour purityillustrated herein provides significant benefits, particularly inrespect of augmented reality applications that use high resolutionarrays of LEDs to form displays in close proximity to users. Further,beneficially, the use of epitaxially grown layers to form colourconversion resonator cavity systems means that the size constraintsimparted by quantum-dot based colour conversion systems are overcome andsmaller light emitting surfaces of light emitting pixels based on microLEDs can be provided, and arrays of light emitting pixels with reducedpixel pitch can be provided.

Whilst FIGS. 1-7 illustrate epitaxially grown colour conversionresonator systems that are formed by sequential growth of layers upon asubstrate, in further examples, a series of layers is epitaxially grownand subsequently bonded to another series of epitaxial layers.Advantageously, by this method, individual colour conversion resonatorcavities, or groups of colour conversion resonator cavities, can beoptimised independently and bonded together, thereby to provide highcrystalline quality colour conversion resonator cavities that areoptimised for their particular wavelength of resonant light.

In FIG. 8 there is shown an alternative embodiment of a colourconversion resonator system 800. The colour conversion resonator system800 comprises the colour conversion resonator cavity 118 epitaxiallygrown atop the partially reflective region 120 and subsequently bondedto the input LED 224 and the substrate device 202 via the bonding layer222. Atop this series of layers there is bonded the partially reflectiveregion 116 and the colour conversion resonator cavity 114 via thebonding layer 850. Additionally, atop this series of layers there isbonded the partially reflective region 112, the colour conversionresonator cavity 110 and, optionally, the partially reflective region108 via the bonding layer 852. Effectively, each colour conversionresonator cavity 110, 114, 118 and its respective partially reflectiveregions are provided separately and bonded together to form thestructure of FIG. 8 . Advantageously, each colour conversion resonatorcavity 110, 114, 118 and its respective partially reflective regions canbe optimised separately prior to being bonded together to form theeventual structure. Such individual optimisation means, for example,that blue and green light emitting structures may be formed based onnitrides materials, whereas red light emitting structures may be formedusing different materials, such as phosphide materials. In furtherexamples, different combinations of materials are used in order toprovide optimised structures for colour conversion and resonance atparticular frequencies of light.

The LED 224 is bonded to the partially reflective region 120 usingdielectric bonding. The surface of the LED 224 that is to be bonded tothe partially reflective region 120 is terminated with a high densityoxide film in order to facilitate such bonding. The surface of thepartially reflective region 120 that is to be bonded to the input LED224 is also terminated with a high density oxide film in order tofacilitate wafer level oxide bonding. Accordingly, the primary lightemitting surface of the LED 224 is placed in close proximity or contactwith the partially reflective region 120 such that light that is outputfrom the LED 224 acts as input light for the colour conversion resonatorsystem 800. Similarly, the colour conversion resonator cavity 118 andthe partially reflective region 116 are terminated with a high densityoxide film to facilitate wafer level oxide bonding. Additionally, thecolour conversion resonator cavity 114 and the partially reflectiveregion 112 are terminated with a high density oxide film to facilitatewafer level oxide bonding.

In further examples, the LED 224 is bonded to the partially reflectiveregion 120 using polymer bonding, such as polyimide bonding. Similarly,the colour conversion resonator cavity 118 is bonded to the partiallyreflective region 116 using polymer bonding, such as polyimide bonding.Further, the colour conversion resonator cavity 114 is bonded to thepartially reflective region 112 using polymer bonding, such as polyimidebonding. In further examples, additional or alternative bondingmechanisms are used in order to attach the corresponding layers.Advantageously, the layers are bonded to form a single device withminimal interface loss of light emission from the LED 224 at theinterface with the colour conversion resonator system 800.

Whilst the layers are shown to be bonded in FIG. 8 with bonding layers222, 850, 852, in further example additional and/or alternative bondinglayers are used to form the structure 800 of FIG. 8 .

In FIG. 9 there is shown a colour conversion resonator system 900comprising the LED 224, the bonding layer 222, the partially reflectiveregion 120, the colour conversion resonator cavity 118, the bondinglayer 850, the further partially reflective region 116 and the furthercolour conversion resonator cavity 114. Each of these layers is grownsequentially and subsequently bonded as described above in FIG. 8 . Atopthis series of layers partially etched layers from the structuredescribed with respect to FIG. 8 is shown. The etched layers are thebonding layer 852, the partially reflective region 112, the colourconversion resonator cavity 110 and the partially reflective region 108.These layers have been etched such that said layers 108, 110 and 112 and852 form a partially overlapping region with remaining layers 224, 222,120, 118, 850, 116 and 114.

In further examples, the colour conversion resonator system 900 isprovided by bonding layers that have already been etched in order toprovide the partially overlapping region. For example, arrays of etchedlayers are provided and bonded together to provide partially overlappingregions corresponding to different light output wavelengths.

In FIG. 10 there is shown a colour conversion resonator system 1000comprising the LED 224, the bonding layer 222, the partially reflectiveregion 120 and the colour conversion resonator cavity 118. These layers,grown and bonded sequentially, as described above, remain unetched. Atopsaid layers 224, 222, 120 and 118, is the bonding layer 850, thepartially reflective region 116 and the further colour conversionresonator cavity 114. The bonding layer 850, the partially reflectiveregion 116 and the colour conversion resonator cavity 114 haveselectively been etched such that layers 114, 116 and 850 form apartially overlapping region with remaining layers 224, 222, 120 and118. As described above, atop layers 114, 116 and 850 is the bondinglayer 852 the further partially reflective region 112, the furthercolour conversion resonator cavity 110 and the further partiallyreflective region 108 such that said layers 852, 112, 110 and 108 form apartially overlapping region with layers 114, 116 and 850. The use of anetch stop (not shown) between the colour conversion resonator cavity 118and the bonding layer 850 facilitates control of the removal of materialby etching. The second etch process forms an exposed light emittingsurface region associated with the colour conversion resonator cavity114 and an exposed light emitting surface region associated with thecolour conversion resonator cavity 118. In further examples, the colourconversion resonator system 1000 is provided by bonding layers that havealready been etched in order to provide the partially overlappingregion. For example, arrays of etched layers are provided and bondedtogether to provide partially overlapping regions corresponding todifferent light output wavelengths.

In FIG. 11 there is shown a colour conversion resonator system 1100where the colour conversion resonator system 1000 described withreference to FIG. 10 has been further processed to provide a firstfurther partially reflective region 526, a second further partiallyreflective region 528, and a third further partially reflective region530. The third further partially reflective region 530 associated withthe colour conversion resonator cavity 110 is provided instead of thepartially reflective region 108 formed in the initial epitaxialstructure. Alternatively, the partially reflective region 108 remains inplace and the third further partially reflective region 530 is notformed in the structure shown at FIG. 11 . The first further partiallyreflective region 526 is formed atop the exposed surface of the colourconversion resonator cavity 118. The second further partially reflectiveregion 528 is formed atop the exposed surface of the colour conversionresonator cavity 114.

Whilst the LED 224 is shown as a single LED, in further examples the LED224 is formed from individually addressable LED devices with individualLED devices corresponding to light output at one or more pixels formedfrom the partially overlapping regions of the colour conversionresonator system 1100. In such a way, high resolution displays can beformed.

In a further example, different combinations of cavities are growntogether and subsequently bonded together. For example, the colourconversion resonator cavity 118 and the colour conversion resonatorcavity 114 can be grown in one step with the partially reflectiveregions 120 and 116. These epitaxially grown layers can then be bondedto the colour conversion resonator cavity 110 and partially reflectiveregions 112 and 108 via a bonding layer. Beneficially, such a processallows the colour conversion resonator cavities 118 and 114 to be grownfrom similar materials to provide high quality cavities and allowscolour conversion resonator cavity 110 to be grown from a differentmaterial which is more optimal for the required wavelength of light inthe colour conversion resonator cavity 110. For example, the colourconversion resonator cavity 118 can correspond to blue wavelength lightand the colour conversion resonator cavity 114 can correspond to greenwavelength light. As such, it may be optimal to grow colour conversionresonator cavities 118 and 114 together from nitride based materials.The colour conversion resonator cavity 110 can correspond to redwavelength light. As such, it may be optimal to grow the colourconversion resonator cavity 110 separately from phosphide basedmaterials.

In order to facilitate the bonding processes described with reference toFIGS. 8 to 11 , handling wafers or growth substrates for the individualcomponents are used and removed at appropriate stages in the deviceprocessing.

Accordingly, the colour conversion resonator system 1100 can be used toprovide an array of pixels, such as a high resolution micro LED array ofpixels that emit light of different wavelengths in a manner similar tothat described with the colour conversion resonator systems of FIGS. 1to 7 .

Whilst methods for forming a colour conversion resonator system aredescribed above with reference to FIGS. 1 to 11 , the skilled personunderstands that in further examples, additional or alternative stepsare used and in yet further examples, some steps are omitted. In furtherexamples, the order of processing steps is altered whilst providing oneor more LED structures in combination with one or more colour conversionresonator cavities to provide improved light emission properties atleast as described herein.

1-20. (canceled)
 21. A color conversion resonator system, comprising: afirst partially reflective region configured to transmit light of afirst primary peak wavelength and to reflect light of a second primarypeak wavelength; a second partially reflective region configured to atleast partially transmit light of the first and second primary peakwavelengths and to reflect light of a third primary peak wavelength; athird partially reflective region configured to at least partiallyreflect light with the third primary peak wavelength; a first colorconversion resonator cavity arranged to receive input light with thefirst primary peak wavelength through the first partially reflectiveregion and to convert at least some of the light of the first primarypeak wavelength to provide light of the second primary peak wavelength,wherein the first color conversion resonator cavity is arranged suchthat the second primary peak wavelength resonates in the first colorconversion resonator cavity and resonant light with the second primarypeak wavelength is output through the second partially reflectiveregion; and a second color conversion resonator cavity arranged toreceive input light comprising the second primary peak wavelengththrough the second partially reflective region and to convert at leastsome of the second primary peak wavelength to provide light of the thirdprimary peak wavelength, wherein the second color conversion resonatorcavity is arranged such that the third primary peak wavelength resonatesin the second color conversion resonator cavity and resonant light withthe third primary peak wavelength is output through the third partiallyreflective region, wherein the first color conversion resonator cavityand the second resonator cavity are arranged partially to overlap toprovide a non-overlapping portion and an overlapping portion thereby todefine a first light emitting surface and a second light emittingsurface respectively, wherein the first light emitting surface isarranged to provide resonant light of the second primary peak wavelengthand the second light emitting surface is arranged to provide resonantlight of the third primary peak wavelength.
 22. The color conversionresonator system according to claim 21, wherein the color conversionresonator system is a monolithic color conversion system.
 23. The colorconversion resonator system according to claim 21, wherein the firstpartially reflective region and the second partially reflective regionare separated by a distance of (N+1) multiplied byθ_(converted)/2n(θ_(converted)), wherein N is an positive integernumber, θ_(converted) is the second primary peak wavelength andn(θ_(converted)) is the effective refractive index of the materialseparating the first partially reflective region and the secondpartially reflective region, thereby to define the length of the firstcolor conversion resonator cavity and/or wherein the second partiallyreflective region and the third partially reflective region areseparated by a distance of (N+1) multiplied byθ_(converted)/2n(θ_(converted)), wherein N is an positive integernumber, θ_(converted) is the third primary peak wavelength andn(θ_(converted)) is the effective refractive index of the materialseparating the second partially reflective region and the thirdpartially reflective region, thereby to define the length of the secondcolor conversion resonator cavity.
 24. The color conversion resonatorsystem according to claim 21, wherein the color conversion resonatorsystem comprises a first LED arranged to control light emission from thefirst light emitting surface and a second LED arranged to control lightemission from the second light emitting surface.
 25. The colorconversion resonator system according to claim 21, wherein the thirdpartially reflective region is further configured to reflect light witha fourth primary peak wavelength, the color conversion resonator furthercomprising: a fourth partially reflective region configure to at leastpartially reflect light with the fourth primary peak wavelength; and athird color conversion resonator cavity arranged to receive input lightcomprising the third primary peak wavelength through the third partiallyreflective region and to convert at least some of the third primary peakwavelength to provide light of the fourth primary peak wavelength,wherein the third color conversion resonator cavity is arranged suchthat the fourth primary peak wavelength resonates in the third colorconversion resonator cavity and resonant light with the fourth primarypeak wavelength is output through the fourth partially reflectiveregion, wherein the third partially reflective region and the fourthpartially reflective region are separated by a distance of (N+1)multiplied by θ_(converted)/2n(θ_(converted)), wherein N is an positiveinteger number, θ_(converted) is the fourth primary peak wavelength andn(θ_(converted)) is the effective refractive index of the materialseparating the third partially reflective region and the fourthpartially reflective region, thereby to define the length of the thirdcolor conversion resonator cavity.
 26. The color conversion resonatorsystem according to claim 25, wherein the second color conversionresonator cavity and the third color conversion resonator cavity arearranged partially to overlap to provide a non-overlapping portion andan overlapping portion thereby to define the second light emittingsurface and a third light emitting surface respectively, wherein thesecond light emitting surface is arranged to provide resonant light ofthe third primary peak wavelength and the third light emitting surfaceis arranged to provide resonant light of the fourth primary peakwavelength.
 27. The color conversion resonator system according to claim21, wherein at least one of the color conversion resonator cavitiescomprises at least one quantum well layer, wherein the at least onequantum well layer is placed to coincide with an antinode of the colorconversion resonator cavity standing wavelength for converted light,thereby enhancing at least one of the intensity, spectral width anddirectionality of output light with the resonant converted wavelength oflight.
 28. The color conversion resonator system according to claim 27,wherein at least one of the color conversion resonator cavitiescomprises at least one absorption layer configured to absorb input lightthereby to enable transfer of energy from the input light wavelengthinto the at least one quantum well layer, wherein the absorption layercomprises a material with a lower energy bandgap than the energy of theinput light.
 29. The color conversion resonator system according toclaim 21, comprising at least one diffusion barrier arranged to reducediffusion of carriers from at least one of the color conversionresonator cavities.
 30. The color conversion resonator system accordingto claim 21, wherein at least one of the color conversion resonatorcavities comprises a quantum well layer, the quantum well layercomprising one or more quantum wells, and a further quantum well layercomprising one or more quantum wells, wherein the separation of thequantum well layer and the further quantum well layer is N multiplied byθ_(converted)/2n(θ_(converted)), wherein N is an positive integernumber, θ_(converted) is the wavelength of the resonant light in thecolor conversion resonator cavity and n(θ_(converted)) is the effectiverefractive index of the material between the quantum well layer and thefurther quantum well layer at the wavelength of the resonant light inthe color conversion resonator cavity.
 31. The color conversionresonator system according to claim 21, comprising at least one furtherpartially reflective region corresponding to at least one of the firstand second light emitting surfaces.
 32. The color conversion resonatorsystem according to claim 21, wherein at least one of the partiallyreflective regions and/or the further partially reflective regionscomprise a distributed Bragg reflector, wherein the distributed Braggreflector is at least one of: a double band distributed Bragg reflector,a conventional distributed Bragg reflector and a vertical stack of twodistributed Bragg reflectors.
 33. The color conversion resonator systemaccording to claim 32, wherein at least one of the partially reflectiveregions comprises a blue wavelength centred low Herpin index DBR or agreen wavelength centred low Herpin index DBR or a red wavelengthcentred low Herpin index DBR.
 34. The color conversion resonator systemaccording to claim 21, wherein at least one of the partially reflectiveregions and the color conversion resonator cavities comprises anepitaxial crystalline layer, wherein the color conversion resonatorsystem comprises at least one of a dielectric material and a III-Vsemiconductor material.
 35. An array of pixels comprising the colorconversion resonator system of claim
 21. 36. The array according toclaim 35, wherein the array comprise a first pixel configured to emitlight of a different wavelength to a second pixel, wherein the firstand/or second pixel comprises a further partially reflective regioncorresponding to its light emitting surface.
 37. The array of pixelsaccording to claim 36, further comprising a third pixel configured toemit light of a different wavelength to the first pixel and the secondpixel.
 38. A method of forming the color conversion resonator systemaccording to claim
 21. 39. The method according to claim 38 comprisingforming at least one of the color conversion resonator cavities on asubstrate.
 40. The method according to claim 39, comprising at least oneof: forming at least one of the color conversion resonator cavities onthe substrate comprises epitaxial growth of a plurality of layers: andforming at least one of the partially reflective regions on thesubstrate, forming at least one of the partially reflective regions onthe substrate comprises sequentially forming at least one of the colorconversion resonator cavities and partially reflective regions on thesubstrate: and bonding the color conversion resonator system to at leastone LED: and bonding together two or more of the first partiallyreflective region; the second partially reflective region, the thirdpartially reflective region, the first color conversion resonator cavityand the second color conversion resonator cavity: and selectivelyetching the color conversion resonator system, thereby to provide thelight emitting surfaces.